A typical mobile phone includes a retractable antenna which is extended during use, either before placing a call or after receiving a call. When waiting for a call, the antenna is usually placed in a retracted position. Therefore, the antenna must be operable in both the extended and retracted positions.
In designing antennas for mobile phones, it is desirable to match the impedance of the antenna to the impedance of the transceiver to maximize power transfer to the antenna. The patent Tsunekawa et al, U.S. Pat. No. 5,374,937 discloses a retractable antenna for a portable mobile phone with means for matching the impedance of the antenna to the impedance of the transceiver both when the antenna is extended and when the antenna is retracted. Keeping the antenna properly matched when retracted allows the phone's receiver to operate at the best possible efficiency under the circumstances, thereby enabling the phone to better detect incoming calls.
While it is possible to operate the phone with its antenna in a retracted position, it is not desirable. Even though the impedance of the retracted antenna can be matched, the radiation pattern of the retracted antenna may be perturbed by the phone's case and by the dielectric load of the user's hand, thereby limiting the transmission efficiency of the phone. In response to this decrease in radiation efficiency, the mobile phone's power control system will cause the phone to increase its transmission power. The net result will be harmless but unproductive dissipation of RF energy leading to early exhaustion of the phone's battery.
Accordingly, there is a need for a mobile phone with a retractable antenna which includes power control means for limiting power when the antenna is in a retracted position to prevent unnecessary dissipation of RF energy.